I've added a bunch of docs with the last commit (https://github.com/clue/graph/commit/45bec93a00b629c0b98ba36c072966a2f1d4fe1a).

I think it should now be clear, so feel free to comment on that basis.

A tree is a connected Graph with no cycles. As such, every tree is a Graph, but not every Graph is a tree. Also, there's a distinction between undirected trees with no root vertices and directed, rooted trees.

(See http://en.wikipedia.org/wiki/Tree_%28graph_theory%29 for more details on the topic)